This invention relates to a signal transmission system, and more particularly to a signal transmission system by which an analogue signal is transmitted in the form converted into a differential pulse code modulated signal train (hereinafter referred to as a "DPCM signal train").
Where an analogue signal, for example, a video signal is transmitted, noise is often carried into the video signal during transmission with the resultant deterioration of picture quality. To avoid this undesirable event, the process has been developed of transmitting a video signal in digital form. However, transmission of the video signal in digital form, for example, in the form of a pulse code modulated (PCM) signal has the drawback that transmission consumes a great deal of time. To resolve this problem, the process has been proposed of transmitting a video signal in the form converted into a differential pulse code modulated (DPCM) signal train through application of a predictive signal train. Transmission of the DPCM signal train not only shortens transmission time, but also offers the later described advantages. Nevertheless, transmission of the DPCM signal train is defective in that once an error arises in the bit arrangement of any of the words constituting the DPCM signal train, all the succeeding DPCM signals are made to have erroneous data. According to the conventional process adopted for elimination of the above-mentioned difficulty, for example, transmission of a video signal in the form converted into the DPCM signal, a train of DPCM signals was transmitted by inserting a PCM signal into said train during each period of scanning one line, thereby correcting the bit error of the respective DPCM signals. Yet, deterioration occurred concentratedly in the right side portion of a reproduced picture. Further, the predictive signal was inserted into a train of DPCM signals, as is well known, at a uniform period. Since, however, each word constituting the predictive signal train had almost twice as many bits as those of each word constituting the DPCM signal train, data of one word constituting the DPCM signal train was lost in that section of said DPCM signal train where the predictive signal was inserted. Therefore, a limitation was imposed on the section of the DPCM signal train where the predictive signal was to be inserted or the frequency of inserting the predictive signal in a transmission signal train. Moreover, picture quality was deteriorated in that section of the DPCM signal train where the predictive signal was inserted.
Where a static picture was transmitted, it was possible to insert the predictive signal without losing the data of a word constituting the DPCM signal train. Still in this case, the problem was raised that an amount of data to be transmitted increased by the amount of data represented by the inserted predictive signal.
It is accordingly the object of this invention to provide a signal transmission system which enables a predictive signal to be inserted into a DPCM signal train more frequently than has been possible in the past without losing data denoted by the DPCM signals and increasing a total amount of data to be transmitted.